Generally, in order to continuously vary the intensity of a light source in a display environment, an incandescent source is employed to which a variable voltage supply is connected. The use of these devices in displays causes a problem for the display's designer in that considerable amounts of heat generated by the incandescent lamps utilised therein must be taken into account in making the design. The heat generated by incandescent lamps must be dissipated otherwise overheating occurs and lamps burn out. The problems associated with incandescent lamps are largely avoided by adopting light sources such as fluorescent tubes. Fluorescent tubes present problems because of flicker when coming on such as to impair their usefulness in a continuously varying display where the flicker destroys the aesthetics of the display and introduce cost penalties in providing components needed for their ignition and control. It is known to continuously vary the intensity of a fluorescent tube over a range of intensities, but flicker free operation from a switched off condition is another problem. As a result, fluorescent tubes have not found application in displays to the same extent as incandescent sources whose intensity is readily and simply variable, and neon tubes which may be switched on without flicker, but are normally operable in a steadily more intense regime built up from a switched off condition.